Viruses must infect host cells to replicate, produce a spreading infection, and cause disease. Infection by enveloped viruses requires binding of the virion to one or more structures on the cell surface (Flint and McKeating, 2000). The initial step may be a low affinity non-specific binding (Barth et al., 2003). Subsequently, the virus binds with high affinity to primary receptors, and then in some cases to secondary receptors or co-receptors (Bartosch et al., 2003; Hsu et al., 2003; Roccasecca et al., 2003; Cormier et al., 2003;_Pohlmann et al., 2003; Zhang et al., 2004). The cell surface binding steps can be associated with a variety of structural rearrangements of the virion surface proteins and changes in protein-protein interactions between the viral surface proteins (Jardetsky and Lamb, 2004; Modis et al., 2004; Bressanelli et al., 2004; Gibbons et al, 2004). The latter steps can expose the fusion peptide, a hydrophobic domain of a viral glycoprotein that is able to interact with cell membranes (Flint et al., 1999; Allison et al., 2001). In some cases, the binding of the virus to the cell surface receptors triggers uptake of the virus through endocytic, or similar vesicular pathways (Garry and Dash, 2003; Jardetsky and Lamb, 2004). Exposure to more acidic conditions in the vesicles can trigger conformational changes in the viral surface proteins, including those that expose the fusion peptide (Kuhn et al., 2002; Lescar et al., 2001). For most viruses, binding to the cellular receptor is primarily the function of one viral surface protein, whereas fusion of the viral and cellular membranes is primarily the function of another viral surface protein. An example of a virus with separate receptor binding and fusion protein is HIV. The receptor binding protein of HIV is the surface glycoprotein (SU; gp120) and the fusion protein is the transmembrane glycoprotein (TM;gp41) (Kwong et al., 1998; Gallaher et al., 1987; 1989). Most viruses with class I fusion proteins in which the fusion peptide is located at or near the amino terminus, for example retroviruses, orthomyxoviruses, paramyxoviruses arenaviruses and coronaviruses, use one protein for receptor binding and another for fusion (Wilson et al., 1981; Gallaher et al, 1996; 2001). Alphaviruses, which have a class II fusion protein with an internal fusion peptide, also use one protein principally for receptor binding and another for fusion of the viral and cellular membranes (Straus and Straus, 1994). The envelope (E) protein encoded by members of the flavivirus genus of the Flaviviridae, has an internal fusion peptide, but serves both receptor binding and fusion function (Allison et al., 2001).
Hepatitis C virus encodes two envelope glycoproteins, E1 (gp35) and E2 (gp70), both with C-terminal transmembrane anchor domains (Flint and McKeating, 2000). E2 interacts with several cell surface proteins (CD81, SR-BI and L-SIGN) suggesting that it is the receptor binding protein of HCV (McKeating, 2004). The function of E1 is less clear and may act to chaperone E2 (Flint et al., 1999; Garry and Dash, 2003). Synthetic peptides corresponding to hepatitis C virus E2 can block infection mediated by hepatitis C virus. Structural determinations of the hepatitis C virus E2 allow the identification of several heretofore unknown features of hepatitis C virus E2 for drug and vaccine development.
The Flavivirus family includes a variety of important human and animal pathogens. Hepatitis C virus (HCV) is the leading viral cause of chronic hepatitis, cirrhosis, liver failure, and hepatocellular carcinoma (Poynard et al., 2003). In the United States alone, an estimated 4 million people are infected with HCV. This is approximately four times the number infected by HIV. Each year in the US, 30-50,000 new HCV infections occur, and about 15-20,000 people die. Moreover, these numbers are expected to increase dramatically given that a substantial portion of HCV infected individuals show little or no response to the only currently approved therapeutics (i.e. treatment with interferons and/or ribavirin). HCV infection is spread primarily through needle sharing among drug users, although there is some risk from accidental needle sticks, blood products before 1992, chronic blood dialysis, and frequent sexual contact. Current treatments for HCV using ribavirin and interferon cost ˜$8,000 to $20,000 per year, and are only partially successful in about half of patients treated. Overall, about 80% of HCV carriers suffer chronic liver inflammation and cirrhosis, of these 25% will develop end stage liver disease or hepatocellular carcinoma (HCC) (Colombo, 2000). End stage HCV disease is the most frequent indication for liver transplants and this costs $250,000 to $300,000. Better drugs to treat HCV infection and an effective vaccine to prevent HCV infection are urgently needed.